While an aircraft is taxiing, it is generally steered by the wheel set carried by the nose (or auxiliary) landing gear, and fitted to the bottom end of the telescopic leg of said landing gear. Steering by means of this wheel set thus requires the end rod of said leg to be drivable in rotation about its own axis, which rod is also axially slidable relative to a strut that contains, amongst other things, a shock absorber.
Landing gear is usually constituted by a leg essentially comprising a tubular strut hinged to the structure of the aircraft (which structure is the part that is driven to retract or extend the landing gear, and has landing gear drive members, side-brace members, and locking members coupled thereto), and a rod that slides axially inside the strut (said rod carrying the axle of the wheel set and co-operating with the shock absorber that is housed inside the strut).
It is thus possible to use landing gear in which the strut is not merely a force-transferring member, but serves also to contribute to guiding and shock absorbing functions for the rod which slides directly inside the strut.
In this case, a sleeve is provided mounted to rotate on the outside of the strut, the sleeve being connected by means of a scissor linkage to the bottom end of the sliding rod.
To rotate the rod about its axis, the mechanism normally implemented includes at least a steering actuator disposed transversely to the axis of the leg, which actuator has its cylinder hinged to the strut and its rod hinged to the rotary sleeve. For example, the state of the art is illustrated in the following documents: GB-A-901 836 and US-A-2 930 547.
The cylinder of the, or each, actuator is thus disposed between two parallel plates that project laterally from the strut in order to obtain a hinge for said cylinder about an axis parallel to the axis of the leg. The rotary sleeve is then necessarily disposed between the top plate(s) and the bottom plate(s), which puts a de facto limit on the height of said rotary sleeve, which sleeve is consequently implemented in the form of a cylindrical collar.
Under such circumstances, that arrangement can be considered to be restrictive insofar as the rotary sleeve runs the risk of being poorly guided (because of its limited height) and because the scissor linkage must be relatively long, thereby, de facto reducing its stiffness, and making the telescopic leg more sensitive to vibration. In addition, safety considerations cause manufacturers to dispose the scissor linkage in such a manner that its top branch is horizontal when the landing gear leg is under static load, and as a result the two branches of the scissor linkage are generally of different lengths.